It is known in the art of manufacturing composite structures that a composite structure may be fabricated using numerous different techniques such as an autoclave process (e.g., using a vacuum bag) or a closed, or partially, or mostly closed mold process (e.g., using a hot press). It is also known that an optical fiber may be embedded within the structure during fabrication as discussed in U.S. Pat. No. 5,399,854 entitled "Embedded Optical Sensor Capable of Strain and Temperature Measurement Using a Single Diffraction Grating", to Voto et al. In particular, it is known to place an optical fiber having one or more gratings embedded therein between plies or lamination layers of the structure (or lay-up). The layers typically comprise reinforcing filaments surrounded by thermal setting epoxy resin. The lay-up is then consolidated in a process which applies pressure and heat to the lay-up, such as an autoclave or hot press, which causes polymerization, curing, and consolidation of the structure.
Also, the embedded fiber must be protected from breakage at the point where the optical fiber enters the composite structure. One such technique which discusses fiber entry into a composite structure in an autoclave process is described in Italian Patent No. RM93A000253, entitled "Method for Embedding Optical Fiber Sensors into Composite Structures", to P. Ferraro, et al. In that process, the lay-up is placed on a lower supporting surface and an upper entry point of the fiber is surrounded with a thick rubber strain relief layer.
Then, using an autoclave process that employs vacuum bagging, a thin plastic layer (or membrane or bag) is placed over one side of the lay-up and sealed to a support surface upon which the lay-up rests. A vacuum is drawn in the volume in which the lay-up is encapsulated between the bag and the supporting surface such that the bag collapses onto the outer surface of the lay-up, thereby applying a predetermined amount of pressure onto the lay-up. At the entry point of the optical fiber, the bag forms a dome-shaped contour containing the thick rubber at the entry point of the optical fiber, thereby allowing the thick rubber to cure (concurrently while the structure itself is curing) on the surface of the structure to create a seal at the location where the optical fiber enters (and/or exits) the lay-up.
The resultant seal configuration has a thick, e.g., about 0.1 inches(") thick, dome-shaped rubber interface on the surface of the lay-up which is bonded to the optical fiber at the point that the fiber enters the composite structure. Such a rubber interface seals the entry point of the optical fiber to eliminate resin flow through the rubber interface, as well as provides a pliable strain-relief interface between the optical fiber and the composite structure to reduce the possibility of fiber damage or breakage at the entry point due to fiber bending.
While applying such a thick rubber interface works well in an autoclave process, such an interface does not work when the composite structure is manufactured using a hot press (i.e., compression-molding with a closed or mostly closed molding tool).
More specifically, a hot press with closed or mostly closed tools exerts much more force on the lay-up than an autoclave process, thereby causing the resin to secrete from (or leak or bleed out of) the lay-up through the fiber entry hole in the lay-up and around the fiber. Also, if the thick rubber interface of the prior art is placed over the fiber entry holes, and the hot-press tool is pressed around that hole, resin will squeeze through the fiber entry point in the lay-up and through the fiber hole in the rubber interface. This creates a resin-coated stiffened section of optical fiber at the fiber entry point where stress concentrations from the fiber bending leads to fiber breakage, thereby making the rubber seal stress-relief ineffective.
Thus, it would be desirable to obtain a rubber interface at the entry point of the optical fiber to a composite structure which, when the structure is consolidated using a compression processes with a closed or mostly closed molding tool (e.g., a hot press), retains a rubber seal stress-relief interface at the fiber entry point.